Modelsim: Error: (vsim-3033)... Instantiation of 'MUT' failed. The design unit was not found

Question

I'm getting the vsim-3033 error in ModelSim when I try including a sub module into a testbench for simulation. All code compiles fine (according to modelsims 'check marks'). The code here is obviously Verilog. I've seen the question "answered" a couple of times - however the explanation at this stage goes over my head.

The exact error is as follows:

vsim -voptargs=+acc work.NGateTB
# vsim -voptargs=+acc work.NGateTB 
# Loading work.NGateTB
# ** Error: (vsim-3033) C:/Users/user/Desktop/testitems/ngateTB.v(7): Instantiation of 'MUT' failed. The design unit was not found.
# 
#         Region: /NGateTB
#         Searched libraries:
#             C:/Users/user/Desktop/testitems/work
# Error loading design

I'm new to ModelSim and I have no idea how to fix it and the issue I'm having is that a lot of the explanations are too "technical" at this stage for me to follow as I'm relatively new to this environment. (please - explain what I'm doing wrong in a way a six years old would understand - and how to fix the issue - and more importantly what it is caused by).

I know this is basically a problem related to "finding the called module" - however I do not know how to fix it. I've tried adding the project root directory into the (project window) -> left click -> properties -> Verilog & SystemVerilog -> include directory...

The code is as follows:

module ngate(A, B);
  input A;
  output B;

  assign A = !B; 

endmodule

the test bench code is:

`include "ngate.v"

module NGateTB();
  reg A;
  wire B;

  MUT ngate (A, B); 

endmodule

When I comment out the MUT ngate(A,B); line the issue disappears. So I know this has to do with something in how ModelSim does includes in simulation. (And not to mention the code basically isolates this as the only cause as far as I can tell). The only other issue is the include syntax perhaps - however I am not sure on this. All files are located in the 'root' of the project space - with the only subfolders being those generated by modelsim in a "work" folder.

When I simulate the ngate module by itself - I have no issues - and it works fine. Currently both items are in the top level of the design with no subfolders (virtual or otherwise). It's only when I include a submodule into another module does an issue appear.

I am using modelsim Altera starter edition 10.1.b. - if that matters.

I hate to admit it - but the IDE has me stumped here.

tim tim · Accepted Answer · 2020-03-28T23:18:44

The code should be:

module ngate
(
    input A,
    output B
);

    assign B = !A;  // A and B were back-to-front.

endmodule

The test bench code should be:

`timescale 1ns/100ps
module NGateTB();

    reg A;
    reg B;  // Need reg not wire.

    ngate MUT (.A(A), .B(B));  // MUT and ngate were back-to-front and signals weren't connected using named association.

    always
    begin
        #10 A = 1'b1;
        #10 A = 1'b0;
        #10 A = 1'b1;
        #10 A = 1'b0;
        #10 A = 1'b1;
        #10 A = 1'b0;
        #10 A = 1'b1;
        #10 A = 1'b0;
        #10 A = 1'b1;
        #10 A = 1'b0;
        $stop;
    end

endmodule

Hope that helps.

Modelsim: Error: (vsim-3033)... Instantiation of 'MUT' failed. The design unit was not found

1 Answers